Branch and link (BL) for subroutines in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to call a subroutine using the branch and link instruction.

ARM Architecture

BL [1]

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Amain

Bend

Cloop

Dsubroutine

Attempts:

3 left

💡 Hint

Common Mistakes

Using a label that is not a subroutine.

Confusing BL with a simple branch instruction (B).

The BL instruction branches to the label subroutine and saves the return address in the link register.

2fill in blank

medium

Complete the code to return from a subroutine using the link register.

ARM Architecture

MOV PC, [1]

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ALR

BR0

CSP

DR1

Attempts:

3 left

💡 Hint

Common Mistakes

Using a general-purpose register instead of LR.

Forgetting to move the link register value to PC.

To return from a subroutine, the program counter (PC) is set to the value in the link register (LR), which holds the return address.

3fill in blank

hard

Fix the error in the code to correctly call the subroutine named 'process'.

ARM Architecture

[1]

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ABL process

Bprocess

CBL_process

Dprocess()

Attempts:

3 left

💡 Hint

Common Mistakes

Adding parentheses to the label.

Using an incorrect label name.

The instruction B process is a simple branch and does not save the return address. To call a subroutine and save the return address, use BL process.

4fill in blank

hard

Fill both blanks to complete the subroutine call and return sequence.

ARM Architecture

BL [1]
MOV PC, [2]

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Ahandler

BLR

CR0

Dstart

Attempts:

3 left

💡 Hint

Common Mistakes

Using a general-purpose register instead of LR for return.

Using a label that is not a subroutine for BL.

The BL handler calls the subroutine named handler. The instruction MOV PC, LR returns from the subroutine by loading the return address from the link register.

5fill in blank

hard

Fill all three blanks to create a subroutine call, save a register, and return correctly.

ARM Architecture

PUSH { [1] }
BL [2]
POP { [3] }
MOV PC, LR

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AR4

Bcompute

DR5

Attempts:

3 left

💡 Hint

Common Mistakes

Not saving and restoring registers properly.

Using different registers for push and pop.

Before calling the subroutine compute, the register R4 is saved on the stack with PUSH {R4}. After the subroutine returns, R4 is restored with POP {R4}. Finally, MOV PC, LR returns control to the caller.